For Canon EOS 1D/MkII/MkIIN/MkIII and EOS 1Ds/MkII/MkIII digital SLR cameras, the Ec-L Focusing Screen (Cross-split Image) makes it easier for the user to get a clear focused picture using manual focusing.
The screen has a 3mm diameter split-image spot, for precise pinpoint focusing on subjects with distinct contours.
Using the central split-image rangefinder, turn the focusing ring until the split image becomes a sharp, clear, complete and undivided image.